We have identified a gene termed CAS which encodes a protein that is localized to microtubules and to the mitotic spindle. CAS is the mammalian homologue of CSE1, a yeast gene which controls mitosis. Several lines of evidence suggest CAS has a similar role in mammalian cells. CAS is located at 20q13, a region that is amplified in some breast, colon and uroepithelial cells. We are investigating the role of CAS in these types of tumors. Because CAS expression is high in proliferating cells, anti-CAS antibodies can be used as a proliferation marker in leukemia and lymphoma cells. Another interesting feature of CAS is that it is abundantly expressed in several nonproliferating tissues. We have found these tissues have spliced forms of CAS mRNA which may confer different functions in these tissues.